Local news network KBS reported that The Seventh Air Force said the U.S. Ambassador to Seoul, Mark Lippert visited Osan Air Base in Pyeongtaek and took an hour-long flight on an F-16 fighter jet above Seoul.

A North Korean foreign ministry spokesman denounced the ambassadors actions, calling it a “vicious act.”

“At this point when the heavy black clouds of nuclear war have gathered over the Korean peninsula it is a very significant development in U.S. North Korea policy that the U.S. Ambassador in South Korea took a flight on a fighter jet,” the spokesman said in an interview with state-controlled KCNA. “It cannot be anything else but an inconceivably evil, vicious act that could only be the work of an insane madman.”

The spokesman said that future events such as the flight could lead to “unprecedented action.”

“We are carefully watching such movements from the United States and taking precautions. We will strengthen our nuclear deterrent to protect our nation’s sovereignty and people’s survival from the U.S. nuclear threat,” the spokesman said.